1. Field of the Invention
This invention relates to a water-based coating material having corrosion resistance composed mainly of an organic synthetic resin having a carboxyl group and zinc oxide particles. More specifically, this invention relates to a water-based coating material that can reduce usage of zinc oxide particles.
2. Description of the Related Art
In recent years, in paint used for coating a surface of a vehicle and the like, shift to water base paint has been advanced in order to reduce a volatile organic compound (VOC) from an environmental viewpoint. In addition, in anticorrosive paint, it is required to depart from paint using a corrosion inhibitor which contains a harmful heavy metal such as a conventional chromium-based corrosion inhibitor and a lead-based corrosion inhibitor from a viewpoint of global environmental protection. Japanese Patent Publication No. (Heisei) 11-241048 shows an invention of a water base paint composition as an example of water base paints having corrosion resistance without using a corrosion inhibitor which contains such a harmful heavy metal.
The Japanese Patent Publication No. (Heisei) 11-241048 discloses the invention of the water based paint composition containing 0 to 50 parts by weight of at least one chromium-free anticorrosive pigment selected from zinc compounds and molybdic acid compounds and 5 to 50 parts by weight of at least one type of calcium ion-exchanged amorphous silica fine particles relative to 100 parts by weight of the total amount of a film-forming resin component comprising 50 to 100 parts by weight of at least one water soluble resin selected from water-soluble alkyd resins and water-soluble epoxy ester resins, 0 to 50 parts by weight of at least one water-soluble acrylic resin and 0 to 30 parts by weight of at least one water-soluble melamine resin. This allows for providing a water base paint that can form a film having an excellent anticorrosion property on an aluminum-coated steel sheet without using a corrosion inhibitor which contains a harmful heavy metal.
However, in the techniques described in the above Japanese Patent Publication No. (Heisei) 11-241048, the film being formed therein is not so tough that it has disadvantages of being easy to peel and fragile even though it has the excellent anticorrosion property. Thus, in paint containing an organic synthetic resin having a carboxyl group and zinc oxide particles, the fact that the carboxyl group forms a three-dimensional structure and a tough film by reacting with zinc oxide particles has attracted attention.
As an example of paints containing such an organic synthetic resin having a carboxyl group and zinc oxide particles, Japanese Laid Open Patent Publication No. 2006-291014 describes an invention on an anticorrosive paint wherein it permits preserving a metallic luster of a laminar film and obtaining an anticorrosive paint having a high adhesive property to a plastic material by containing metallic oxide particles having a particle size of 20 to 50 nm, an acrylic-type resin and a silane coupling agent.
However, in the techniques described in the above Japanese Laid Open Patent Publication No. 2006-291014, ultrafine metallic oxide particles having a particle size of 20 to 50 nm (which is an average particle size calculated based on an arithmetic average) are used to preserve the metallic luster of the laminar film and ensure transparency, and thus manufacturing cost is considerably increased, and the laminar film is fragile because of difficulty in forming a thick film and it is unable to obtain a tough film therein. On the other hand, when using zinc oxide particles having a particle size about 0.1 to 1 μm (which is an average particle size measured with a laser diffraction-type particle size distribution measuring apparatus), the carboxyl group increases an area to react with zinc oxide particles and it is required to increase the additive amount of zinc oxide particles, and thus there is a problem of cost increase also.
Accordingly, an object of the present invention is to provide a water-based coating material composed mainly of an organic synthetic resin having a carboxyl group and zinc oxide particles which permits obtaining a tough film having an excellent corrosion resistance even if an additive amount of zinc oxide particles is reduced less than that of conventional ones and permits obtaining a tougher film having a much excellent corrosion resistance by coupling more carboxyl groups to a surface of zinc oxide particles when making the additive amount of zinc oxide particles equal to that of conventional ones.